Design of Lead(II) Metal-Organic Frameworks Based on Covalent and Tetrel Bonding

Three solid materials, [Pb(HL)(SCN)₂]CH₃OH (1), [Pb(HL)(SCN)₂] (2), and [Pb(L)(SCN)]_n (3), were obtained from Pb(SCN)₂ and an unsymmetrical bis-pyridyl hydrazone ligand that can act both as a bridging and as a chelating ligand. In all three the lead center is hemidirectionally coordinated and is thus sterically optimal for participation in tetrel bonding. In the crystal structures of all three compounds, the lead atoms participate in short contacts with thiocyanate sulfur or nitrogen atoms. These contacts are shorter than the sums of the van der Waals radii (3.04-3.47 Å for Pb⋯S and 3.54 Å for Pb⋯N) and interconnect the covalently bonded units (monomers, dimers, and 2D polymers) into supramolecular assemblies (chains and 3D structures). DFT calculations showed these contacts to be tetrel bonds of considerable energy (6.5-10.5 kcal mol^-1 for Pb⋯S and 16.5 kcal mol^-1 for Pb⋯N). A survey of structures in the CSD showed that similar contacts often appear in crystals of Pb^II complexes with regular geometries, which leads to the conclusion that tetrel bonding plays a significant role in the supramolecular chemistry of Pb^II.